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DATE 2011-07-01

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Key: archive Value: 2011-07-01

Key: id Value: 540634

MESSAGE
DATE 2011-07-07
FROM Ruben Safir
SUBJECT Subject: [NYLXS - HANGOUT] (fwd) iostream manipulator construction and design
From owner-hangout-outgoing-at-mrbrklyn.com Thu Jul 7 02:07:07 2011
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From: Ruben Safir
To: hangout-at-nylxs.com
Subject: [NYLXS - HANGOUT] (fwd) iostream manipulator construction and design
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-- forwarded message --
Path: reader1.panix.com!panix!not-for-mail
From: Ruben Safir
Newsgroups: comp.lang.c++
Subject: iostream manipulator construction and design
Date: Wed, 6 Jul 2011 02:55:09 +0000 (UTC)
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Xref: panix comp.lang.c++:1087303

I've covered the iostream operations in Stroustrup but there is a section
that I'm failing to understand in section 21.4.6.1 (of the third edition
of C++ - The Programming Language) which I hope that someone can help
me decipher. The section discusses Manipultors that take arguments and
Stroustrup states


"Manipulators that take arguments can also be useful. For example, we
might want to write

cout << setprecision(4) << angle;

to print the value of the floating-point variable angle with four
digits.

To do this, setprecision must return an object that is initialized by 4
and that calls cout::setprcision when involked. Such a manipulator is a
function object thatr is involed by << rather than by (). The exact
tyoe of function object is implementation-defined, but it might be
defined like this:

struct smanip{
ios_base& (*f)(ios_base&, int); //function to be called
int i;

smanip(ios_base& (*ff) (ios_base&, int), int ii) : f(ff, i(ii){}
};

template
ostream& operator<<(ostream& os, smanip& m)
{
return m.f(os, m.i);
}

The smanip constructor stores its arguments in f and i and the
operator<< calls f(i). We can now define setprecision() like this:

ios_base& set_precision(ios)base& s, int n) //helper
{
return s.setprecision(n); //call member function
}

inline smap setprecision(int n)
{
return smanip(set_precision, n);
}

We can now write:
cout << setprecision(4) << angle;
"


What don't understand a couple of things here, and this might be an
accumulation of increasing confusion on my part as I've continued
through this chapter.

First, I just don't understand what the problem is that is being solved.
Why can't a manipulator just take an argument of an int and return a
base_stream object, like the manipulators that don't take an argument?

Secondly, the exqmple completely befuddles me. I'm guessing that don't
understand the order of operations of

cout << setprecision(4) << angle;

is ostream<< called first and then the function setprecision?

-- end of forwarded message --

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